Microsporidia
are eukaryotic, unicellular organisms belonging to the phylum Microspora. All
microspoidia are obligate, spore-forming, intracellular parasites that invade
vertebrates and invertebrates. A characteristic feature of microsporidia is the
polar tube or polar filament found in the spore used to infiltrate host cells.
They are widely distributed in nature with over 1200 species characterized.
However, microsporidia have only recently been documented to parasite humans,
and more research is needed to understand this emerging infectious disease.
Infection in humans, Microsporidiosis, is primarily found in patients with
compromised immune systems, especially those infected with Human
Immunodeficiency Virus (HIV) or have undergone organ transplants.Some species, however, have also been
known to parasite those with health immune systems. Beyond the human realm,
Microsporidia are important parasites in fisheries, veterinary medicines and
pest management.

Classification and taxonomy

The
scientific classification of Microsporidia has evolved through time with
growing scientific research in the area, and the specifics are still currently
debated. Initially thought to be a protozoan (kingdom Protista), recent studies
using DNA techniques indicate phylum Microspora should be classified under the Fungi kingdom or at least as a sister kingdom to Fungi.
The class, order and family within the Microspora phylum are also frequently
revised and debated (traditional taxomony given at the family level bloew).
Traditionally, species were identified by observing the physical
characteristics of the spore, life cycle and relationship with the host cell.
However, recent scientific studies using genetic tools (namely ribosomal RNA
sequencing) have challenged this approach and suggest genetic markers a more
correct method for scientific classification. More research is still needed to
better understand the origins of microspora and of individual species.Despite all this, there are now over
1200 species identified in 143 genera. Currently, at least 14 species in 8
genera are known to infect humans.

The
Phylum Microspora was discovered in the late 1800s, but the first human case
was described only in 1959 in a Japanese child. The rise in microsporidiosis is
associated with the arrival and spread of HIV; microsporidiasis is primarily
found in patients with AIDS or are otherwise immuno-compromised (like organ
transplant patients). However, at least three species of Nosema and one of Brachiola have been documented in immuno-competent
patients. Microsporidia are considered casual, accidental or opportunistic
agents in humans.

Agent Morphology

Microsporidia
are primitive eukaryotes with well defined nuclei and plasma membrane but lack
some typical organelles found in more typical eukaryotes mainly mitochondria,
stacked golgi and peroxisumes. Microsporidia spores are all round and oblong
and those associated with human infection tend to be about 1-4 µm in size (an
important feature for diagnosis as some species are often mistaken for
bacteria). All have a characteristic coiled polar tube, tubule or filament,
layered polarplast, a posterior vacuole (thought to function as golgi) and a
protective exospore made of proteins and chitin. Chitin is responsible for the
sporesŐ high environmental resistance. Below are two diagrams (3D and 2 D)
illustrating typical microsporidia spores.

During
infection, the cell membranes of the spore (also called the sporoplasm) is
injected into a host cell though osmosis pressure.After the infection, the microsporidium relies on the host
cell for energy and begins to multiply inside the host cell cytoplasm. Species
differ in their relationships with host cell; some species alter host cell
functioning to induce more nutrient absorption and cell growth to accommodate
the agent. Microsporidia may reproduce sexually or asexually. In asexual
reproduction, nuclear division takes place and form one or more pairs of
nuclei, and cellular division may isolate the nuclei or pair them in a
diplokaryon arrangement. Sexual reproduction is not well understood but it
thought to involve autogamous fusion and reorganization of genetic material.

Life Cycle

The
life cycle of microsporidia varies between species, but can be generalized in
the following diagram.

First,
(1) the environmentally
resistant, infectious spore is ingested or otherwise contracted. This
environmental stimulus activates germination in the spore, releasing the polar
tubule through eversion.(2) The spore then inject the polar tubule into a
host cell and (3) release
their sporoplasms into host cells. At this stage, the sporoplasms usually
become meronts, cells with loosely organized organelles enclosed in a simple
plasma membrane.(4) The microsporidia meronts then multiply either
in contact with the host cytoplasm (as with E. bieneusi) or within a parasitophorous vacuole (in E.
interstinalis). (5) They then undergo sporogony to further divide
and form sporoblasts, preparing the cell for thick exospore layers
characteristic of mature spores. The spores can either be dispersed freely
within the host cell cytoplasm or packaged in sporophorous vesicles (or
pansporoblast membranes). This characteristic can be taxonomically important in
distinguishing between species. Finally, (6) when spores completely fill the host cell, the
plasma membrane is affected and releases the spores to its surroundings. The
spores may then infect other surrounding cells, be transported to new sites
within the host, or be excreted in feces or urine to infect other hosts.

The
exact incubation period for microsporam spores is unknown, but the spores are considered
extremely resistant and thus assumed to persist in the environment for long
periods of time.

Reservoirs and Vectors

Microsporidia
spores are ubiquitous and are capable of infecting any animal cell including
those of insects, fish, mammals and even other parasites! They are most
commonly found to infect anthropods. Many of the 14 species infecting humans
are also found in a number of wild and domesticated mammals (e.g. rabbits,
mice, puppies, kittens, etc.). In fact, the first case of E. cuniculi was described in rabbits in 1923. There are no
vectors for microsporidiosis in the standard sense (infected insects may infect
humans if they are ingested, but this is neither necessary nor ubiquitous).

Transmission

The
transmission of microsporidia is still unclear, but the most common way is
thought to involved inhaling, ingesting or otherwise contracting spores (for
example ocular or sexually transmitted). Spores of microsporidia may also be
transmitted in water as species of Encephalitozoon, Enterocytozoon and
Vittaforma have been documented in water sources (Dowd et al. 1998).
Significant contact with infected animals may also transmit the disease (zoonoic
infection) but cases are rare.

Clinical Presentation in Humans

Chronic
diarrhea and wasting are the most common symptoms of microsporidiosis, but
different species invade different sites including the cornea, binary track and
muscles. Thus, the symptoms of microsporidiosis varies greatly depending on the
site of infection.

The methods of diagnosis typically involve identifying
spores in feces, urine, other bodily fluid or body tissues. Transmission electron microscopy (TEM)
is the gold standard for identifying specific species and diagnosing
microsporidiosis but is often too expensive and time consuming. Alternatives
include light microscopy using various stains including gram stains (microsporidia
spores are gram-positive and stain dark violent and become readily visible
under the microscope), modified trichrome stains (e.g. trichrome blue), Warthin-Starry
silver stains, Giemsa, and chemofluorescent agents like Calcoflur. E. bieneusi measures between .8-1.5 µm while B. algerae, Encephalitozoon spp, V.
corneae and Nosema spp. measure 1.5-4 µm under the microscope. Immunofluorscent assays (IFA) and molecular
techniques are emerging methods for diagnosis.

Treatment, Therapy and Management

Treatment
with albendazole is most common for all species and is coupled with topical Fumagillin
for ocular infections (see table of key drug treatments below) However, most
drug treatments will not fully eradicate the parasites. New, more effective
drugs for microsporidiosis are still being discovered and tested. For example, Nikkomycin
Z (NIK-Z), a drug that inhibits chitin synthesis, has been shown to be
affective against a host of fungal pathogens. It proved successful in laboratory
tests on Encephalitozoon species, but has yet to be tested in vivo.

Further
management of the disease is often needed. In general, symptoms should be
treated if possible. Patients with chronic or severe diarrhea should take care
to replenish electrolytes and fluids regularly and maintain nutritional
intakes.

As
mentioned, microsporidia are extremely widespread. They infect nearly every
organism on earth from honey-bees and silkworm to mammals and birds. Relatively
little is known about the epidemiology of microsporidia, as transmission and
infection pathways are still somewhat unclear. Though active microsporidia
spores have been found in water sources in developed and developing nations,
microsporidiosis remains primarily a disease of HIV and AIDS patients. Microsporidia
has been reported to infect 39% of AIDS patients with diarrhea and 30% of AIDS
patients with Cryptosporidium.
Despite our relatively recent
discovery of this pathogen, the infection among AIDS patients is remarkable and
the parasite will be of growing importance in the future as HIV continues to
spread and more research is undertaken to understand the role microsporidia
play in the human health.

Geography

Microsporidia
has a worldwide distribution, affecting both developing and developed nations,
but proper diagnosis remains difficult, especially in developing nations. Below
is a map illustrating the few countries where microsporidiosis has been
formally documented.

Map made by Lily Cheng, May 21th, 2006

Public Health and Prevention Strategies

In
recent years, the US Environmental Protection Agency (EPA) has listed microsporidia
in the EPA Candidate Contaminate List (CCL), deeming it an emerging water-borne
pathogen needing monitorial attention. Filtrating water supplies remains the
best preventative strategies available. Measurement and filtration techniques
for microsporidia spores remain rudimentary and underdeveloped, though the
scientific community is actively trying to amend this knowledge gap.

There
are currently no vaccines available or being explored for microsporidia
infection.

Although
microsporidia infection in humans mostly occur in patients with compromised
immune systems, the further spread of AIDS worldwide increases our need to
understand and manage microsporidia for the near future. As more research is
done on this class of organisms, we find their prevalence increasing in human
patients. This is indeed an emerging infectious disease.